A sulfur removal process can extract mercaptan from a hydrocarbon stream to a caustic stream. In a sulfur extraction unit, alkali extracts mercaptan from a hydrocarbon stream. These mercaptides may then be oxidized to disulfides by adding air and catalyst, and running the stream through an oxidizer.
In sulfur extraction units, a regenerated alkali stream is often used. The mercaptides in the alkali may be converted in the presence of oxygen to disulfides in an oxidizer. These three phases, spent air, lean alkali, and disulfide oil, can then be separated in a disulfide separator. Frequently, the alkali may further be contacted with a hydrocarbon to separate more disulfide oil from the alkali, requiring another vessel. This vessels may require increased plot space. Moreover, the disulfide oil can be sent from the disulfide separator to a filter or water wash to remove entrained alkali prior to being sent to downstream processing.
In some processes, the alkali regeneration section operates at a pressure between about 280 kPa (g) to about 410 kPa (g) (about 40 to about 60 psig) and a temperature between about 43° C. and about 54° C. (about 110 to about 130° F.). However, the alkaline needs to be returned to the extractor at a pressure of about 690 kPa (g) to about 2410 kPa (g) (about 100 to about 350 psig). Consequently, there are alkali circulating pumps positioned downstream of the disulfide separator vessel.
Wash oil is often necessary to obtain low sulfur liquefied petroleum gas (LPG) product from the sulfur extraction process (e.g., less than 5 wppm S). The use of lighter hydrocarbons (e.g., butane or lighter) as the wash oil source is precluded because much of it would vaporize at this operating pressure and temperature. However, in some locations, such as a gas plant, the only source of wash oil would be these lighter hydrocarbons.
Thus, it would be desirable to provide a caustic regeneration process which would allow the use of lighter hydrocarbon as the wash oil.